There has been a recent emergence of highly pathogenic avian influenza (HPAI) viral strains in poultry and their subsequent transmission to humans in southeast Asia, which has raised concerns about the potential pandemic spread of lethal disease. In 1997, highly pathogenic avian influenza H5N1 influenza virus was transmitted from poultry to humans in Hong Kong, resulting in eighteen infected people and six deaths, and reemerged in 2003 causing two similar cases with one fatality. H5N1 influenza virus infections in family clusters have raised the additional possibility of human-to-human transmission. As human exposure to and infection with H5N1 influenza viruses continues to increase, so, too, does the likelihood of the generation of an avian-human reassortant virus that may be transmitted efficiently within the global human population, which currently lacks H5N1 specific immunity. Such reassortant events between avian-human and swine-human influenza A viruses have been associated with at least the 1957 and 1968 influenza pandemics and potentially the 1918 pandemic. Concern over the potential for the generation of a pandemic H5 strain and its concomitant morbidity and mortality are spurring the search for an effective vaccine.
Influenza viruses are RNA orthomyxoviruses and consist of three types, A, B, and C. Influenza A viruses infect a wide variety of birds and mammals, including humans, horses, pigs, ferrets, and chickens. Influenza B and C are present only in humans. Animals infected with influenza A often act as a reservoir for the influenza virus, by generating pools of genetically and antigenically diverse viruses which are transmitted to the human population. Transmission may occur through close contact between humans and the infected animals, for example, by the handling of livestock. Transmission from human to human may occur through close contact, or through inhalation of droplets produced by coughing or sneezing.
The outer surface of the influenza A virus particle consists of a lipid envelope which contains the glycoproteins hemagglutinin (HA) and neuraminidase (NA). The HA glycoprotein is comprised of two subunits, termed HA1 and HA2. HA contains a sialic acid binding site, which binds to sialic acid found on the outer membrane of epithelial cells of the upper and lower respiratory tract, and is absorbed into the cell via receptor mediated endocytosis. Once inside the cell, the influenza virus particle releases its genome, which enters the cell nucleus and initiates production of new influenza virus particles.
NA is also produced, which cleaves sialic acid from the surface of the cell to prevent recapture of released influenza virus particles. The virus incubates for a short period, roughly five days in a typical case, although the incubation period can vary greatly. Virus is secreted by affected cells approximately one day prior to the onset of the illness, and the resultant illness typically lasts three to five days. Typical symptoms include fever, fatigue, malaise, headache, aches and pains, coughing, and sore throat. Some symptoms may persist for several weeks post infection.
Different strains of influenza virus are characterized primarily by mutations in the HA and NA glycoproteins, and thus HA and NA identity are used to identify viral subtypes (i.e., H5N1 indicates HA subtype 5 and NA subtype 1). Influenza vaccines often target the HA and NA molecules. Conventional influenza virus vaccines often utilize whole inactivated viruses, which present the appropriate HA and/or NA molecules as antigens to prime the cellular and humoral immune systems. Alternatively, recombinant forms of the HA and NA proteins or their subunits have been used as vaccines. However, influenza is an RNA virus and is thus subject to frequent mutation, resulting in constant and permanent changes to the antigenic composition of the virus. In one example, small, minor changes to the antigenic composition are often referred to as antigenic drift. In another example, Influenza A viruses are also capable of “swapping” genetic materials from other subtypes in a process called reassortment, resulting in a major change to the antigenic composition of the virus. Because the immune response against the viral particles relies upon the binding of antibodies to and cell-mediated immune system component recognition of the HA and NA glycoproteins, frequent changes to the composition and structure of glycoproteins reduces the effectiveness of the humoral and cell-based immune responses against influenza viruses over time, eventually leading to a lack of immunity. The ability of influenza A to undergo a rapid antigenic shift can often trigger influenza epidemics due to this lack of preexisting immunity to the new strain.
The lengthy development time and limited production capability of conventional inactivated influenza vaccines could severely hinder the ability to control the pandemic spread of avian influenza through vaccination. Thus, there is a need in the art for a method of quickly developing and mass producing large quantities influenza vaccine.